Disorders of the brain are serious medical conditions causing disability and diminished quality of life. Neurological damage is largely irreversible and thus early diagnosis and close monitoring are critical to the successful treatment of patients. Alzheimer's disease (AD) is a neurodegenerative disease associated with progressive memory loss and cognitive dysfunction. It is associated with abnormal clumps (amyloid plaques) and tangled bundles of fibers (neurofibrillary tangles) in the brain, both of which are considered signs of AD. An estimated 4 million Americans have AD. By the year 2030 approximately 1 in every 80 persons in the U.S. will have AD. Familial Alzheimer's disease (FAD) is known to be inherited. In affected families, members of at least two generations have had the disease. FAD is rare, accounting for less than 1% of all cases of AD. FAD has an earlier onset, i.e., about 40 years of age and can be observed to run in families.
Early-onset Alzheimer's disease (EOAD) is a rare form of Alzheimer's disease in which individuals are diagnosed with the disease before age 65. Less than 10% of all Alzheimer's disease patients have EOAD. Younger individuals who develop Alzheimer's disease exhibit more of the brain abnormalities that are normally associated with Alzheimer's disease. EOAD is usually familial and follows an autosomal dominant inheritance pattern. To date, mutations in three genes including amyloid precursor protein (APP) on chromosome 21, presenilin 1 (PSEN1) on chromosome 14 and presenilin 2 (PSEN2) on chromosome 1 have been identified in families with EOAD. Mutations in the APP, PSEN1 and PSEN2 genes account for about 50% of the disease. Most of the pathogenic mutations in the APP and presenilin genes are associated with abnormal processing of APP, which leads to the overproduction of toxic A˜−1-42. Down syndrome patients, who have three copies of chromosome 21 which includes the APP gene, begin to develop the characteristic senile plaques and tau tangles at the ages of 30 and 40 (Kamboh, Annals of Human Genetics 68:381-404, 2004).
Late-onset Alzheimer's disease (LOAD) is the most common form of Alzheimer's disease, accounting for about 90% of cases and usually occurring after age 65. LOAD strikes almost half of all individuals over the age of 85 and may or may not be hereditary. It is a complex and multifactorial disease with the possible involvement of several genes. Genome-wide linkage or linkage disequilibrium studies on LOAD have provided informative data for the existence of multiple putative genes for AD on several chromosomes, with the strongest evidence on chromosomes 12, 10, 9 and 6. LOAD cases tend to be sporadic, wherein there is no family history of the disease. Genetic susceptibility at multiple genes and interaction between these genes as well as environmental factors are most likely responsible for the etiology of LOAD. Twin data on incident cases indicates that almost 80% of the LOAD risk is attributable to genetic factors. The Apolipoprotein E (APOE) gene on chromosome 19q13 has been identified as a strong risk factor for LOAD. In fact, the APOE-ε4 allele has been established as a strong susceptibility marker that accounts for nearly 30% of the risk in late-onset AD. More specifically, three variants of APOE, encoded by codons 112 and 158, have been found to modify the risk of LOAD. As compared to the common APOE-ε3 allele (codon 112=Cys and codon 158=Arg), the APOE-ε4 allele (codon 112=Arg and codon 158=Arg) increases the risk of AD, while the APOE-ε2 allele (codon 112=Cys and codon 158=Cys) decreases the risk of AD. The effect of the APOE-ε4 allele is dose related, wherein one or two copies of the APOE-ε4 allele are associated with 3-fold or 15-fold risk, respectively. However, the effect of the APOE-ε4 allele on AD risk appears to decline with increasing age (Kamboh, 2004, supra).
From the time of diagnosis, people with AD survive about half as long as those of similar age without dementia. Medicare costs for beneficiaries with AD were $91 billion in 2005 and may increase to as much as $160 billion in 2010. Finding a treatment that could delay the onset by five years could reduce the number of individuals with AD by nearly 50 percent after 50 years. Drug development for AD is very active and sensitive diagnostic and screening technologies could identify patients for therapy and monitor their response. Improved diagnostic tools for AD would thus be a significant advancement to drug development for this disease and would also provide a way to guide therapeutic decision making thus improving outcomes and reducing unnecessary exposure of patients to costly medications with unwanted side effects.